Conventionally, press apparatuses (electric press, electro press) have been known for carrying out a press-fitting operation, etc. in assembly of small components, etc. as shown in FIG. 1 (For example, see Patent Literature 1.).
In a press-fitting operation of such a press apparatus (electric press, electro press), in some cases, it is desired to make a certain determination or stop the press-fitting operation at a point of time (position) when a condition(s) is changed during the press-fitting.
Herein, the above described change of the condition appears as a bend point in a graph of position/pressurization-load as shown in FIG. 12. In order to detect this bend point, there is a method that focuses on the slope thereof. More specifically, a point of time (position) at which the slope exceeds a certain setting is obtained. This method has been conventionally realized as functions such as differential judge and differential stop.
The above described method is an effective method under predetermined conditions. However, on the other hand, if it focuses on the “slope”, in other words, the change rate of the load, the bend point cannot be precisely detected in some cases. For example, since the values of the slopes are varied depending on individual parts, a slope that serves as a determination reference cannot be set in some cases.
In such a case, it is conceivable to capture an essential characteristic of “inflection point” that “the change rate of the change rate of load” has an extremum. However, calculating “the change rate of the change rate of load”, in other words, obtaining a second-order differential value is to obtain a second-order difference if positions and load values are digitized discrete values.